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Tohatsu MFS6DD (6HP 4-Stroke) - Dies Under Load / Above Idle

D

djcanfield

New member
Hi everyone,

I'm troubleshooting a 6HP Tohatsu 4-stroke outboard (Model: MFS6DD, Serial: 035980RB) that starts easily and idles cleanly, but bogs and dies immediately when advancing throttle under load. Overall, the engine is in like-new cosmetic condition.

Here is what I have already inspected, cleaned, or replaced:
• Carburetor: Dismantled and cleaned existing carb, including a thorough visual inspection/cleaning of the main jet and emulsion tube.
• Fuel Pump: Replaced existing pump with a new non-OEM pump.
• Fuel Filter: Replaced with new filter.
• Fuel Supply: Tested on both the internal tank and an external tank (same behavior on both).
* The choke or pulsing the bulb during starvation helps but does not solve the issue.

Despite these steps, the motor continues to stall under load.

Questions:
1. Is it common for the internal micro-passages in these small MFS6DD carbs to remain blocked even after manual cleaning/jet inspection, warranting a complete OEM carb replacement (OEM #3R4032000M)?
2. Could a non-OEM fuel pulse pump lack sufficient pulse pressure at higher RPMs to keep the float bowl filled under load?
3. Are there any known ignition/CDI high-speed coil breakdowns on these models that trigger only under load?

Thanks in advance for any advice or diagnostic pointers!
 
Yes, yes, no. People rarely clean the carburetor correctly so I would start there. Remove and disassemble the carburetor. Then place the body and the metal parts into a 1 gallon can of carburetor acid which you can get from any auto parts store. Let it sit for 3 hours in the acid. Remove the body and parts and wash clean with water and then blow dry with compressed air. Reassemble and reinstall. There would be no conceivable reason to use any fuel pump other than the factory one. Electrical breakdowns are virtually unknown, but it is always a possibility. Using the service manual you would get the values and procedure to test the components.
 
I took your advice and bought some cleaner from O'Reillys. After soaking it, I washed it out thoroughly with hot water, but I didn't have any compressed air with me. I dried it with a hairdryer and let it sit overnight. When I reinstalled it today, there was a definite improvement. It runs for about a minute under load before it starves and dies. It will run on idle indefinitely. Do I need to repeat this process and maybe clean it out more thourghly at the end with canned carb cleaner and canned air. Thanks so much for your help!
Tahatsu Dying (YouTube).
 
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